lO 



THE GERM CELLS: MITOSIS, MATURATION AND FERTILIZATION 



Ovulation and Menstruation— When the ovum is ripe, the Graafian folUcle 

 is large and contains fluid, probably under pressure. The ripe follicles form 

 bud-like projections at the surface of the ovary (Fig. 6), and at these points 

 the ovarian wall has become very thin. It is probable that normally the bursting 

 of the Graafian folhcle and the discharge of the ovum are periodic and associated 

 with the phenomena of menstruation, as maintained by Fraenkel and Villemin. 

 That ovulation or discharge of the ovum from the ovary may occur independently 

 of the menstrual periods has been proven by the observations of Leopold and 

 of Ravano. Also in young girls ovulation may precede the inception of men- 

 struation and it may occur in women some' time after the menopause. 

 Ovum 



Follicle cells 



Zona pclluihlii 

 Fig. 7. — Immature follicle containing six ova. From the ovary of a young monkey. X 430. 



At birth, or shortly after, all of the ova are formed in the ovary of the female 

 child. Hensen estimates that a normal human female may develop in each ovary 

 200 ripe ova. Most of the young ova, which may number 50,000, degenerate and 

 never reach maturity. At ovulation but one ovum is normally ripened and dis- 

 charged from the ovary. Several ova, however, may be produced in a single 

 folhcle in rare cases. Such multiple folhcles have been observed in human ovaries 

 and are of frequent occurrence in the ovary of the monkey (Fig. 7). 



The Spermatozoon. — The male cell or spermatozoon of man is a minute cell 

 0.055 mm. long, specialized for active movement. Because of their active move- 



